1. Field of the Invention:
The present invention relates to a safety shackle, and more particularly to such a shackle for use in hoisting a structural element of buildings or the like which is installed in an elevated position, the shackle positively engaging the element until released therefrom by repeated oscillatory movements of a ratchet mechanism from a position remote from the location at which the element is engaged by the shackle.
2. Description of the Prior Art:
In constructing buildings and the like it is frequently necessary to raise a structural element of the building to a location wherein the element is rested in its installed disposition. Normally, the element has a bore extending through it and it is connected to a hoisting cable provided with a clevis by a pin extended through said bore and through bores in the clevis aligned therewith. Before hoisting, a rope or the like is connected to the pin outwardly of the clevis for drawing the pin from the bores to release the clevis from the structural element when the element is rested in said disposition. For a number of reasons, this conventional method of hoisting and releasing such elements is extremely dangerous at worst and highly inconvenient at best.
This method is dangerous because the pin is only retained in the bores by friction due to weight of the element being hoisted subjecting the pin to shear stress. Therefore, the pin can easily become displaced allowing the element to fall and injure persons and/or property below. A primary cause of such displacement is snagging of the rope. The rope is easily snagged since it necessarily dangles from the element during hoisting so that the rope can be grasped to withdraw the pin from the bores when the element is emplaced. If the rope becomes snagged at any point during hoisting, continued hoisting draws the pin from the bores and disengages the element from the hoisting cable.
This danger is greatly increased by the necessity for the rope being substantially longer than the vertical distance between the pin and the ground surface when the structural element is emplaced. Since the pin extends horizontally, the rope must extend in angled relation from the pin to obtain enough purchase to withdraw the pin despite friction between it and the bores. This excess length of the rope, of course, increases the danger of snagging. To reduce this danger, the element must first be hoisted vertically a distance such that the rope clears all portions of a building being constructed, then swung horizontally, and finally lowered to its installed elevation. This excess hoisting not only wastes time, but requires the use of a crane having a longer boom than would otherwise be necessary. The risk of tipping the crane over during hoisting is, of course, increased by the use of a longer boom than would otherwise be required.
When, as intended, the pin does not become dislodged before the element is emplaced, it is often extremely difficult to remove the pin and release the hoisting cable for further use. This difficulty can arise because the rope is kept short to avoid the previously described snagging and hoisting difficulties and because of friction between the pin and the walls of the bores. In many cases the pin cannot be withdrawn even by jerking the rope. In such an event, it is necessary for a person to ascend the partially completed structure to a precarious position to drive the pin out with the attendant danger of falling while ascending and driving the pin.